Again though, this has big issues. Why are 90%+ of the Starfleet crew (and cadets) we see human?
I think this is just a conceit of TV that the crews we see are mostly-Human; there's a point where I just suspend disbelief. I can rationalize it by assuming that most of the crews we don't see are majority-alien. DIS established that Starfleet in the 2250s had around 7,000 ships; presumably it has even more in the TNG era. That's a lot of crews we haven't seen that could be majority-alien.
We can also assume that a fair number of Human-looking background characters might actually be from Human-like species such as Betazoids, Ardanans, Argelians, Elasians, Risians, etc.
Why do the Vulcans seemingly have their own fleets?
Why wouldn't they? State defense forces, answerable solely to their state's government and not to the U.S. federal government, exist today in real life. State police forces exist even though the FBI exists. Why wouldn't Federation Member States retain their own local fleets in addition to having Starfleet cover the entire UFP?